The words “criticality” and “re-criticality” have been used extensively in the media coverage. Criticality is a nuclear term that refers to the balance of neutrons in the system. “Subcritical” refers to a system where the loss rate of neutrons is greater than the production rate of neutrons and therefore the neutron population (or number of neutrons) decreases as time goes on. “Supercritical” refers to a system where the production rate of neutrons is greater than the loss rate of neutrons and therefore the neutron population increases. When the neutron population remains constant, this means there is a perfect balance between production rate and loss rate, and the nuclear system is said to be “critical.” The criticality of a system can be calculated by comparing the rate at which neutrons are produced, from fission and other sources, to the rate at which they are lost through absorption and leakage out of the reactor core. A nuclear reactor is a system that controls this criticality or balance of neutrons.

The power of a reactor is directly proportional to the neutron population . If there are more neutrons in the system, more fission will take place producing more energy. When a reactor is starting up, the neutron population is increased slowly in a controlled manner, so that more neutrons are produced than are lost, and the nuclear reactor becomes supercritical. This allows the neutron population to increase and more power to be produced. When the desired power level is achieved, the nuclear reactor is placed into a critical configuration to keep the neutron population and power constant. Finally, during shutdown, the reactor is placed in a subcritical configuration so that the neutron population and power decreases. Therefore, when a reactor is said to have “gone critical,” it actually means it is in a stable configuration producing a constant power.

A reactor is maintained critical during normal power operations. In other systems, such as a spent fuel pool, mechanisms are in place to prevent criticality. If such a system still achieves criticality, it is called “re-criticality”. Boron and other materials, which absorb neutrons, are in place to make sure that this re-criticality does not occur. The added neutron absorbers substantially increase the rate of loss of neutrons, to ensure a subcritical system.

Most types of light water reactors (like the BWRs in Japan) use water to not only cool the reactor, but to also slow down neutrons. In these systems, slower neutrons cause the majority of fission reactions. Therefore, if the water boils off, neutrons will not slow down as much and the probability of fission reactions and power decreases, thus putting the nuclear system in a subcritical state.

If water heats up and vaporizes in a BWR reactor or spent fuel pool without cooling, the temperature increase of the water and eventual vaporization of water will tend to place the system in a subcritical condition. There are also large amounts of boron in these systems such as the control rods of the reactor, and various kinds of boron in the spent fuel pools. Additionally, steel structures supporting the spent fuel in the pool are sometimes made out of borated steel, which also contains large amount of boron. Even if the fuel does melt, the new geometric configuration will likely not be favorable for slowing down neutrons, so re-criticality is unlikely, even if water should be reintroduced to the system.